The present invention relates to an X-Y address type solid state image pickup device in which unit pixels each including an active device for converting a signal charge obtained through photo-electric conversion by a photo-electric conversion device into an electrical signal and outputting the electrical signal are arranged in a matrix form, and a method of producing the same.
Solid state image pickup devices are generally classified into a charge transfer type solid state image pickup device represented by a CCD image sensor and an X-Y address type solid state image pickup device represented by a CMOS image sensor. Of the two types. the X-Y address type solid state image pickup device will be described referring to FIG. 9 which shows an example of the sectional structure of the CMOS image sensor taken as an example.
As is clear from FIG. 9, the CMOS image sensor has a construction in which a pixel portion 100 for photo-electric conversion of incident light and a peripheral circuit portion 200 for reading a signal by driving pixels, processing the signal and outputting the processed signal are integrated on the same chip (substrate). Transistors constituting the pixel portion 100 and transistors constituting the peripheral circuit portion 200 have a part of wiring in common.
The pixel portion 100 includes a photo-diode 102 provided on the surface of an N type silicon substrate 101 having a thickness of about several hundreds of μm, and a color filter 105 and a micro-lens 106 arranged on the upper side of the photo-diode 102 with a wiring layer 103 and a passivation layer 104 therebetween. The color filter 105 is provided for obtaining color signals.
In the pixel portion 100, transistors and wirings are present between the photo-diode 102 and the color filter 105. Therefore, in order to enhance the ratio of the incident light on the photo-diode 102 to the incident light on the pixel portion 100, namely, numerical aperture, the incident light is focused on the photo-diode 102 through the gaps between the wirings by the micro-lens 106.
However, in the related art of the pixel structure in which the incident light is taken into the photo-diode 102 through the wiring layer 103 as mentioned above, a portion of the light focused by the micro-lens 106 is scattered by the wirings, resulting in various problems as follows.
(1) The amount of light is reduced by the portion scattered by the wirings, so that sensitivity is lowered.
(2) The portion of light scattered by the wirings enter into photo-diodes in the adjacent pixels, resulting in color mixture.
(3) Characteristics are lowered due to limitations on the basis of wiring, such as the limitations that a wiring cannot be disposed on the upper side of the photo-diode 102 and a thick wiring cannot be laid, and it is difficult to miniaturize the pixels.
(4) The light is incident skewly on pixels and the ratio of the light portion scattered to the entire amount of the incident light is increased in a peripheral area, so that dark shading occurs heavily at the pixels in the peripheral area.
(5) It is difficult to produce a COMS image sensor by an advanced CMOS process with an increased number of wiring layers, because the distance from the micro-lens 106 to the photo-diode 102 is increased.
(6) A library of advanced CMOS processes cannot be used due to (5) above, a change in layout of the circuit in the library is needed, and an increase in area is caused by limitations on the wiring layer, so that production cost is raised and pixel area per pixel is enlarged.
Further, when light with a long wavelength such as red color light undergoes photo-electric conversion in a P well 107 located deeper than the photo-diode 102 in FIG. 9, the electrons generated diffuse through the P well 107, to enter into photo-diodes at other positions, resulting in color mixture. In addition, if the electrons enter into a pixel shielded from light for detection of black, a black level may be detected erroneously.
Besides, in the CMOS image sensors in recent years, there is the tendency that the functions which have been provided on different chips, such as a camera signal processing circuit and a DSP (Digital Signal Processor), are mounted on the same chip as the pixel portion. Because the process generation is advanced in the manner of 0.4 μm→0.25 μm→0.18 μm→0.13 μm, if the CMOS image sensors themselves cannot cope with these new processes, they cannot share in the benefit of miniaturization, and cannot utilize the rich CMOS circuit library and IP.
However, the degree of multilayer property of the wiring structure advances as the process generation advances; for example, three-layer wiring is used in the 0.4 μm process, and, on the other hand, eight-layer wiring is used in the 0.13 μm process. Besides, the thickness of wiring is also increased, and the distance from the micro-lens 106 to the photo-diode 102 is increased by a factor of three to five. Therefore, with the face side irradiation type pixel structure in which light is led to the light-receiving surface of the photodiode 102 through the wiring layer according to the related art, it has come to be impossible to efficiently focus the light on the light-receiving surface of the photo-diode 102, and, as a result, the above-mentioned problems (1) to (6) have come to be conspicuous.
On the other hand, the charge transfer type solid state image pickup devices include the back side light reception type frame transfer CCD image sensor which receives light from the back side. In the back side light reception type frame transfer CCD image sensor, a silicon substrate is thinned to receive light on the rear side (back side), a signal charge obtained through photo-electric conversion inside silicon is caught by a depletion layer extending from the face side, is accumulated in a potential well on the face side and is outputted.
An example of the sectional structure of a photo-diode in the back side light reception type frame transfer CCD image sensor is shown in FIG. 10. In this example. the photo-diode is composed of a P type region 303 at the surface on the side of an oxide film 302 provided with wirings or the like with respect to the silicon substrate 301, and is covered by an N type well (epi layer) 304 through a depletion layer 305. A reflective film 306 of aluminum is provided on the oxide film 302.
In the case of the back side light reception type CCD image sensor having the above-mentioned structure, there is the problem that the sensitivity to blue light for which absorbance is high is lowered. In addition, the signal charge generated upon photo-electric conversion at a shallow position of the light incident on the rear side diffuses, to enter into photo-diodes in the surroundings at a certain ratio. In addition to these problems, the CCD image sensor is characterized in that the height of the wiring layer need not be enlarged because system-on-chip is not conducted, focusing by an on-chip lens is easy because a light-shielding film can be dropped into the surroundings of the photo-diode owing to an exclusive process, the above-mentioned problems (1) to (6) are not generated, and it is unnecessary to adopt the back side light reception structure. For these reasons, the back side light reception type CCD image sensor is rarely used at present.
On the other hand, in the case of the CMOS image sensor, the process used is one obtained by minor modifications of a standard CMOS process, so that adoption of the back side light reception structure offers the merits that the process is not affected by a wiring step and the newest process can always be used, which merits cannot be obtained with the CCD image sensor. However, as contrasted to the CCD image sensor, the wirings are present in many layers in the form of crossing lines, so that the above-mentioned problems (1) to (6) appear conspicuously as the problems peculiar to the CMOS image sensor (and hence the X-Y address type solid state image pickup device represented by this).